1. Field of the Invention
The present invention relates generally to ovens and, more specifically, to a rotational molding oven able to evenly heat and cool a mold filled with resin materials while rotating the mold to manufacture primarily hollow or partial shell objects wherein the resin is evenly distributed throughout the object.
2. Description of the Prior Art
Numerous methods and apparatuses for forming an object have been disclosed in the prior art. On such example is rotational molding. Rotational molding is a method of manufacture for primarily hollow or partial shell shaped plastic objects. This process utilizes a shell mold having a cavity bounded on five sides by the mold. The sixth side of the cavity is formed by a cover attached by clamps or bolts to one of the adjacent sides. When the cover to the mold is open, a powdered (possibly colored) plastic resin is placed into the cavity of the mold. The cover is then sealed to restrict access to the cavity and the mold is placed in a heated environment in which it is rotated about two axes. The heat causes the resin to melt against the heated inside surface of the mold. The melted resin flows within the cavity to form a viscous membrane conforming to the mold""s inner surface. The mold (and the plastic inside) is then cooled while rotation continues causing the resin to harden in the shape of the cavity filled thereby.
When the hardened resin is cool enough to handle (normally below 150xc2x0 F.) the rotation is stopped and the mold is opened. The hardened resin forming a desired part is removed. The part is then trimmed and cut to the form the desired final contour(s) for the part. These parts are generally of uniform wall thickness, colored throughout, and unstressed, i.e. the parts will not deform if subject to cyclical heat or heating/cooling.
The quality of the part produced is dependent on both the heating cycle and the cooling cycle. The heating cycle is divided into four stages. The initial stage is the warm up of the oven interior and mold support structure to a process temperature for the mold shell. This stage is an unproductive time and may contribute to uneven heating patterns which for the most part are undesirable. Generally, the shorter the warm up cycle, the better and more efficient the process. Excessive heat infusion may cause momentary overheating and/or mold distortion. Therefore, time and temperature control is important. Equally important is the uniformity and rate of convective heat transfer, the higher the gas velocity about the mold, the greater the heat transfer and the more uniform the heating of the mold.
The second stage of the heating cycle is attaining the resin melting temperature within the oven. At this stage, the resin is still predominantly in powder form and heat transfer must be maintained at maximum level. The mold wall temperature must be kept below the rapid oxidation temperature or else discoloration (the first indication of burning) or oxidation will occur. High gas velocities are important during this stage to ensure that no portion of the mold is cooler or hotter than the bulk of the mold and that the initial melt of the resin is uniform.
The third stage starts after the resin begins to melt. At the initiation of the third stage, the majority of the resin is in contact with the mold wall and heat transfer has begun to slow down. The mold wall temperature begins to rise, approaching the oven gas temperature. If left unchecked, the resin in contact with the mold wall may start to discolor. At this stage the oven temperature must be reduced. In some systems this reduction is progressive over the heating cycle. Alternatively, in many ovens in use today, the second stage is ignored and the temperature is held at the third stage temperature limits throughout the process. This prevents burning but also slows the process cycle thereby sacrificing speed of production in order to obtain satisfactory quality for the produced part.
The forth stage is the cool down of the mold. In some ovens the cool down is left to nature, e.g. convective air cooling is applied and the mold cools slowly. Heavier and shielded parts of the mold cool slower than lighter unshielded parts. The large exposed sections of the mold cool quickly. Factoring the irregular cooling during this stage of the overall heating cycle into the manufacturing cycle of a part is an inexact art form. Modern ovens use water spray cooling to quickly cool or de-superheat the mold and mold support structure. Preferably, the spray is in the form of a mist. A spray mist provides an enhanced cooling effect which is more uniform and regulated than convective air heating. Excessive cooling such as water deluge will cause uneven shrinkage of the mold and molded part and may damage some molds. When the plastic part inside of the mold is below the melting/viscous point of the resin, the heating stages are complete.
The cooling cycle occurs over three stages. The initial stage is a continuation of the last stage of the heating cycle. The mold and structural support attachments must be cooled to a point where heat flows out of the mold. Since the resin is a poor conductor of heat, the inner surface of the molded part cools much slower than the surface in contact with the mold. As the cooling continues, the part begins to shrink. Shrinkage will cause some portions of the molded part to detach from the mold wall and these areas will now cool slower than the balance of the molded part. This may cause some degree of distortion. In this stage, excessive rate of cooling causes the part to warp.
The second stage of cooling begins when all of the part has cooled and is released from the mold wall. At this point, the temperature of the part is completely below the viscous temperature. The heat transfer rate is at its slowest at this point due to a lack of part-to-mold contact. The part can now be cooled quickly with little fear of increased deformation. An increased use of water spray is the generally accepted procedure for further cooling the part.
The last stage of cooling occurs after the part has cooled to a point where it could be safely removed from the mold. The part may still be soft and additional cooling may help post-molding operations, otherwise the part is complete and is held waiting for operator attention. This stage is therefore non-critical to the overall process. In interconnected, multiple mold systems, this stage is often required so that other molds can be processed.
Rotation of the mold can include either complete revolutions about two axes or complete revolutions about a single axis with partial revolutions about a second axis. The latter type of rotation is called xe2x80x9crock and rollxe2x80x9d as the partial revolutions are similar to a cradle being rocked. In both cases, the two axes of rotation are mutually perpendicular and horizontal rotation about the vertical axis is not required.
The method of heating the mold may be either direct or indirect. Direct heating by an open flame or radiant panels is not considered here. Heating by an open flame is a very old technique characterized by uneven heating, a potential for flame impingement and low energy efficiency. The cost of equipment is very low. Direct heating using radiant panels is still under development and presents limitations for molds having complex shapes and/or curvatures.
Indirect heating is performed in an oven and is currently the preferred method used by most of the industry. Using a direct flame inside the oven is a special case and is subject to the same quality limitations for open direct flame methods mentioned hereinbefore. The indirectly heated oven is discussed hereinbelow.
Another type of system is based on bi-axial rotation. Bi-axial rotational systems, i.e. ovens with mold handling mechanisms having two axes of complete rotation, are the most popular commercially made ovens. These ovens provide the most universal rotational patterns and are well suited for a large portion, but not all, of the marketplace. Commercial systems of this type can be either shuttle, clamshell or turret styles.
Systems of the turret type have molds mounted on three or more radially displaced hollow, horizontal arms. The arms, which provide the primary axis of rotation, are typically fitted with a right angle drive head located at the end of the hollow rotating arm and inside the oven. The drive head displaces the molds to the side of, and at right angles to the primary axis of rotation. Bevel gears mounted on shafts with bearings in the drive head, driven by a second shaft positioned inside the hollow rotating arm independently drive the mold about an axis perpendicular to the primary axis of rotation. The drive head operates entirely within the hot oven during the heating cycle.
Two to four molds may be mounted on the drive head depending on design and complexity of the drive head and the size of the molds. However, mold size is limited to a fraction less than half the height of the oven. Therefore, bi-axial ovens tend to be large. Mold volume efficiency is limited to less than one-third (⅓) of the oven volume, to allow for complete rotation of the arm about the primary axis with molds attached. The bi-axial ovens are generally indirectly heated by gas fired burners operating under temperature control.
A shuttle machine carries a single hollow rotating arm having an independently driven drive head at one end similar to that of the bi-axial oven and is able to either manually or automatically extract the mold from the oven. Typically, one set of molds is removed to a remote cooling station while the oven is used by a second shuttle to heat a second set of molds. Two cooling stations are thus required.
Clamshell ovens are typically opened at a center parting line and raised off of or away from the molds following the heating cycle. The cooling cycle occurs in open air. The clamshell ovens thus require a high overhead and front swing clearance. The clamshell ovens are often single use type. However, a clamshell oven can be combined with a shuttle oven.
Turret style ovens are popular due to their ease of automation. The three or more hollow rotating arms of the turret style oven are horizontally supported from a central turret. The arms rotate horizontally, about a vertical axis extending through the center of the turret, into the oven carrying the molds into position for the heating cycle and, after the heating cycle, are again rotated horizontally carrying the molds into the cooling station. The third position of the turret style oven serves as a mold loading and unloading station. Each station occupies 120 degrees of an arc having its center at the turret and an overall radius sufficient to encompass the oven. The turret system can cycle each arm based on pre-programmed conditions thereby relieving operators of the need to attend to every cycle change. Loading and unloading of the molds in a turret system is manual. For turret ovens, seals are far more complicated and subject to more leakage and oven volume efficiency is worse for turret ovens than for the two previously mentioned systems. This is due to the curvature of the mold path, the vertical movement of the doors and the fact that, to provide a completely unobstructed swing path, one side of the furnace is horizontally split such that the top part of the oven is hung from the oven structure.
The horizontal rotation of arms and molds make these units large. The ratio of useable floor space to that required for the molds to clear the process stations is low and therefore a great deal of floor space is required to install even a small system.
Another type of oven is the rock and roll oven. The rock and roll oven rotates the mold about a horizontal primary axis. The entire drive and mold assembly is then rocked through a smaller angle, horizontal and perpendicular to the first axis of rotation. The rock angle can be varied from 1 to 30xc2x0 above and below the horizontal and angles of up to xc2x145xc2x0 can be achieved. For some very long parts such as canoes or kayaks, the angle may be limited to about xc2x115xc2x0. The molds can be directly heated such as in open flame systems. However, direct heated ovens have lower energy efficiency, are subject to poor process control and have largely been replaced by indirect heated oven systems.
Rock and roll ovens were once very popular, especially when plastic processors and custom molders were making their own equipment. Typically rock and roll ovens have fewer parts, a simple mold path, a smaller unit footprint and a lower manufactured cost. Maintenance of the rock and roll oven is easy since the parts were small (e.g. one unit-one mold) the ovens used off-the-shelf low horsepower motors and drives and critical components. All components are easily accessible. However, loading and unloading mechanisms for these ovens were not well defined and subsequently, labor content per part manufactured is high. Furthermore, the product quality and product consistency are often below industry requirements.
Lack of cycle control made part processing parameters vary with ambient temperature at the start of any cycle. This means that parts were generally under processed early in the day in a cold oven and overheated later in the day when the oven became hotter. Heating was uneven and circulation within the ovens had low velocity. Many systems had in-cavity gravity burners which subjected the molded parts to moments of direct flame impingement and moments of low heat and had low thermal efficiency. Furthermore, none of the systems were fully automated and therefore the amount of labor required of an operator was typically high and expensive, especially in a labor limited situation. For these reasons the rock and roll systems have never dominated the marketplace.
Rock and roll ovens have a few advantages over the bi-axial systems which are inherent in the operation. Firstly, most parts can, in fact, be adequately molded in a rock and roll environment and parts which have an axis of symmetry such as cylinders, box shapes and cones and have critical thickness requirements at the top or bottom rims may perform better on a rock and roll system. In bi-axial rotation ovens there is a period during the secondary rotation in which one end of the mold points downward. During this period, the resin collects and stagnates because there is no gravity-induced motion of the resin powder within the mold. The end of the part in which the resin collects is therefore too heavy. Orienting the part sideways or at an angle to the axis of rotation to overcome this problem places one side of the mold in a less desirable position with regards to heat transfer. The side which faces the mounting hardware will thus be thinner when the process is complete. The heavy section mentioned above is not eliminated but appears in a less conspicuous side wall area. This is not an improvement.
Rock and roll oven systems can be configured to operate in a smaller foot print with lower head room thereby consuming much less factory real-estate. The smaller size of oven means that the buildings housing the ovens can be shorter and smaller thereby reducing plant overhead costs.
The mechanical mechanisms of the rock and roll oven are generally simpler and easier to maintain than those of large bi-axial ovens. The rock and roll ovens generally have smaller motors, starters, gears and drive linkages. Most of the drive devices are located in easy to maintain locations. This translates directly into lower maintenance costs.
In the common turret style bi-axial oven all parameters are fixed. To optimize oven space, multiple molds are mounted but they all must run under the same process conditions. This causes compromises in process cycle time and quality as all cycles pass through the same oven. Smaller parts are processed inefficiently and larger parts can not be processed. The production mix may leave some cycles empty, these must be run empty, in order to keep the other cycles operating.
In the proposed embodiment of the patent, a system of multiple, single cavity ovens, dependence of one mold upon the requirements of any other is eliminated and compromising of individual process parameters is not required. Use of multiple ovens also permits a larger variety of parts to be made at one time. One mold to one oven means that ovens that are not required are not operated, thus, there are no dead cycles or lost energy. A multiple oven system can each be made up of ovens of different sizes and configurations whereby molds of all sizes can be matched to their optimum oven. A smaller total number of large or special ovens reduces the capital investment and saves energy.
The cost to manufacture these ovens is generally low and often several rock and roll ovens can be purchased for the same cost as one large turret style bi-axial system. The rock and roll ovens do not have to be identical, e.g. a mix of sizes, configurations and features will save initial capital investment.
Numerous other types of ovens have also been provided in the prior art. For example, U.S. Pat. Nos. 4,468,172; 4,632,654; 4,767,321; 5,039,297 5,423,248; 5,443,382 and 5,683,240 all are illustrative of such prior art.
U.S. Pat. No. 4,486,172 to Dunning discloses an oven and method of operation for heating thermoplastic articles. Articles are fed into the oven on a conveyor belt. There is a heating plate directly beneath the article bearing surface of the conveyor belt which heats the supported side of the article. The surface of the article not in contact with the bolt is heated by conventional means within the oven.
U.S. Pat. No. 4,632,654 to Dunning discloses rotational molding apparatus and methods for rotationally molding castable material such as polymers and other materials. In one form, a shuttle or wheeled carriage containing a mold fixture for holding one or more molds, which fixture is both pivotable and rotatable on the carriage, is operable to move along a fixed path into and out of an oven and cooling chamber. Two of such mold fixture containing carriages may be automatically moved between the oven and one or more cooling chambers, alternately to permit the oven and cooling chamber or chambers to be operated substantially all of the time during an operating shift. An automatic programming device or computer operates to control the movement of the carriage or carriages into and out of the oven and cooling chamber, the operations of the fixture rotating and pivoting motors and, if utilized, the operation of automatic mold charging equipment, the cooling chamber water pumps, the oven heating elements, mold opening and closing means, molded article removal equipment, etc. In particular form, mold containing carriages alternately move into the oven and cooling chamber along a single track wherein one carriage is either sidetracked or moved to the side of the single track while the other carriage travels between the oven and cooling chamber.
U.S. Pat. No. 4,767,321 to Chilva discloses a method and apparatus for heating fibers reinforced thermoplastic sheets is disclosed. The apparatus involves use of gas heating ovens adapted to allow several layers of material to be heated continuously, with the conveyors stacked are above the other. Stacking of the heated product can be provided at the oven exit. Provisions for cleaning and diffusing the gases over the work piece are also described.
U.S. Pat. No. 5,039,297 to Masters discloses rotational molding apparatus for molding a kayak and the like in a mold. The apparatus includes an oven having an oven chamber. A frame pivotally supports oven above a ground floor. Mold rotates about a roll axis on a carriage while in oven chamber. Oven pivots about a pivot axis in counter-pivotal movement. Oven pivot axis is spaced a distance xe2x80x9cdxe2x80x9d from roll axis of mold. This causes a pendular motion to be imparted to the mold. Mold thus swings to and fro in an arc as oven pivots. Mold rotates about its roll axis at the same time. A well-controlled, even distribution of plastic material in the mold occurs by this combination of motions. A desired pattern of heat distribution is applied along the length of mold by a series of hot air openings and a like number of openings on an opposing side of a hot air plenum.
U.S. Pat. No. 5,423,248 to Smith et al discloses method and apparatus for heating a product which includes a plurality of tapered ducts in a cabinet above and below a conveyor to form streams which are directed toward the product. Spent air is drawn through return ducts which have intake openings centered between entrance and exit openings in the cabinet and centered between lateral edges of a conveyor and between the tapered ducts to provide a balanced flow of spent air in the cabinet to the return opening. Temperature controlled gas is delivered at an angle through an array of openings adjacent opposite edges of an opening through which a conveyor extends to cause most of the heated air to be drawn to the return duct opening and to maintain internal pressure in the cabinet to prevent ingress and egress of air through the opening.
U.S. Pat. No. 5,443,382 to Tsurumi et al discloses an atmospheric oven containing an atmospheric gas kept at a predetermined purity accommodates a transport for transporting an object to be heated along a predetermined transporting path. A rectangular sectioned tubular body for preventing the gas from flowing outside the oven extends a certain length from an entrance of the oven and an exit of the oven and has a sectional area necessary for passing the object through the tubular body.
U.S. Pat. No. 5,683,240 to Smith et al discloses a method and apparatus for heating a product which includes a plurality of tapered ducts in a cabinet above and below a conveyor to form streams which are directed toward the product. Spent air is drawn through return ducts which have intake openings centered between entrance and exit openings in the cabinet and centered between lateral edges of a conveyor and between the tapered ducts to provide a balanced flow of spent air in the cabinet to the return opening. Temperature controlled gas is delivered at an angle through an array of openings adjacent opposite edges of an opening through which a conveyor extends to cause most of the heated air to be drawn to the return duct opening and to maintain internal pressure in the cabinet to prevent ingress and egress of air through the opening.
While these units may be suitable for the particular purpose to which they address, they would not be as suitable for the purposes of the present invention as heretofore described.
The present invention relates generally to ovens and, more specifically, to a rotational molding oven able to evenly heat and cool a mold filled with resin materials while rotating the mold to manufacture primarily hollow or partial shell objects wherein the resin is evenly distributed throughout the object.
A primary object of the present invention is to provide a rotational molding oven that will overcome the shortcomings of prior art devices.
Another object of the present invention is to provide a rotational molding oven which is able to provide both heating and cooling within a single cavity wherein all cycles begin from the same start-up state with the same preheat cycle thus providing a consistent mechanical and thermal cycle which reduces the number of rejects produced.
A further object of the present invention is to provide a rotational molding oven which is able to provide a short timed period of extra heat input for the preheat cycle and which allows for close control of temperature to a predetermined, variable temperature profile throughout the heating cycle.
A yet further object of the present invention is to provide a rotational molding oven wherein the burner combustion chamber is located external to and to the rear of the oven and provides hot gasses to a plenum located below the cavity and is therefore capable of operating on a variety of fuels.
A still further object of the present invention is to provide a rotational molding oven including an exhaust stack including a volume control damper to control cavity negative pressure and a combustion gas re-circulating fan connected to the combustion chamber to optimize combustion and to reduce tramp air infiltration into the cavity.
A further object of the present invention is to provide a rotational molding oven having an adjoining separately supported drive assembly located to the rear of the cavity for rotating the mold within the cavity about the center of the cavity without vertical translation and wherein all drives and drive train components are external to the cavity.
A further object of the present invention is to provide a rotational molding oven wherein the drive assembly is supported by a carriage which moves up and down along a curvilinear track causing the drive assembly to rotate the mold and which curvilinear track provides for automatic control of the rotational motion including multiple stops and starts at adjustable predetermined tilt angles.
A still further object of the present invention is to provide a rotational molding oven supporting a mold at the center and rotating the mold in centerless fashion about two axes of rotation at the center of the oven wherein the oven will be the least possible size to house the rotating mold.
Another object of the present invention is to provide a rotational molding oven that is simple and easy to use.
A still further object of the present invention is to provide a rotational molding oven that is economical in cost to manufacture.
Additional objects of the present invention will appear as the description proceeds.
A rotational molding oven for forming an object from a mold filled with resin is disclosed by the present invention. The rotational molding oven includes a cavity for retaining the mold, a burner for heating the cavity and melting the resin, a fan for cooling the cavity and hardening the resin, a device for rocking the mold preferably between xc2x11xc2x0 and xc2x145xc2x0 from a horizontal plane extending through the cavity center and a device for rotating (rolling) the mold about an axis running through the cavity center and perpendicular to the aforesaid axis. The rocking device is positioned outside and extending into the cavity, maintaining the mold in a center of the cavity. The rocking performed by the rocking device provides for axial transport of the resin inside the mold. Rocking the mold at the center of the cavity improves thermal flow of heat within the cavity and around the mold thereby increasing thermal efficiency and minimizing the size of the cavity. The rocking device includes a first track positioned outside the cavity, a carriage assembly to run on the first track, a drive assembly connected to the carriage assembly and a guide assembly connected between the first track and carriage assembly for translating up and down the first track, causing the carriage assembly to move therewith and the mold to rock about the axis through the center of the cavity. The carriage assembly is positioned below the guide assembly and remains below the guide assembly through all points of translational movement. The rotational drive assembly includes a spindle bearing connected to the carriage assembly and a spindle arm extending from the spindle bearing and through a side of the cavity for releasably connecting to the mold for rotating the mold through complete 360xc2x0 rotations in both a clockwise and counterclockwise direction. A cooling system comprised of a cooling fan, cooling fan inlet damper and cool air inlet damper is also positioned about the cavity for cooling and hardening the melted resin in the mold.
To the accomplishment of the above and related objects, this invention may be embodied in the form illustrated in the accompanying drawings, attention being called to the fact, however, that the drawings are illustrative only, and that changes may be made in the specific construction illustrated and described within the scope of the appended claims,